Smelting reduced iron ore pellets in the blast furnace



United States Patent 3,282,678 SMELTlNG REDUCED IRON ORE PELLETS IN THE BLAST FURNACE Norwood B. Melcher and Morris M. Fine, Minneapolis, Minn, and Philip L. Woolf, Pittsburgh, Pa., assignors to the United States of America as represented by the Secretary of the interior N0 Drawing. Filed Jan. 16, 1964, Ser. No. 338,268 2 Claims. (Cl. 75-41) The invention herein described and claimed may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of royalties thereon or therefor.

This invention relates to production of pig iron in a blast furnace.

Improvement in the quality of blast furnace raw materials (burden) in recent years has resulted in substantial increases in productivity. Among modern burden materials are iron ore pellets which may contain about 63-65 percent iron, usually as ferric oxide. These materials cannot be greatly improved by physical means because the content of extraneous gangue material in the iron ore concentrates (from which pellets are manufactured) is usually quite low.

Partial reduction of iron ore prior to use in blast furnaces or steel furnaces has also been suggested, as disclosed in Patent No. 2,523,138. However, such processes have been only partially successful in achieving increased productivity of pig iron or steel.

It is therefore an object of the present invention to provide a method of increasing blast furnace productivity.

It is a further object of the invention to lower the coke rates of blast furnaces.

It has now been found that these objects may be accomplished by means of a process in which prereduced iron ore pellets, i.e., pellets in which the ore has been partially reduced to metallic iron, are employed as at least part of the iron ore portion of the blast furnace burden. The use of such prereduced pellets has been found to re sult in a sharp increase in pig iron production rate and an equally sharp decrease in the quantity of coke required per unit of pig iron produced.

Furthermore, it has been found that a linear relationship exists between the percent of prereduction of the ore and both the production rate and decrease in coke consumption. These relationships are expressed by the following equations:

where P is the percentage increase in production rate of pig iron as compared to that resulting from smelting unreduced iron ore pellets, C is the savings in coke (pounds) per ton of pig iron produced as compared to that resulting from smelting unreduced pellets and R is the percent of prereduction, i.e., the percent of oxygen removed from the ore prior to use in the blast furnace burden. Knowledge of the relationships expressed by the above equations is obviously very advantageous in operation of the blast furnace using prereduced ore pellets since they make possible prediction of the magnitude of improvement (increase of production) as well as the manner in which the furnace must be operated (amount of coke required) to achieve the benefits of the use of prereduced pellets.

The means employed for prereduction of the pellets is not critical and may be accomplished by a variety of means conventionally employed in reduction of iron ores, e.g., various reducing agents such as coke, coal, CO, H etc., may be employed and various temperatures may be used depending on the type of reductant employed and 3,282,573 Patented Nov. 1, 1966 the amount and physical state of the pellets. The extent of prereduction may also vary considerably, generally being from about 30 to percent.

The ore pellets need not all be reduced to the same extent. As will be seen in the examples below, the burden may comprise mixtures of pellets of different degrees of reduction or mixtures of reduced pellets and nonreduced pellets.

The pellets may be prepared by first hardening (induration) by heating to a high temperature. The hardened iron ore pellets are subsequently reheated and reduced with solid or gaseous reductants. It has been found, however, that a particularly good burden material results from preparing the pellets according to a process in which induration (of green pellets) and reduction are accomplished simultaneously. According to this process the carbonaceous reductant is not incorporated in the pellet but is external thereto and functions both as a reductant and to cushion the uncured spheres until hardening takes place. The following example illustrates this method of preparation of the preferred pellets.

EXAMPLE 1 Preparation of prereduced pellets parts of iron ore concentrate, containing 71()% moisture was thoroughly mixed with /2 to 1 /2 parts bentonite binder and fed to a rotary disc pelletizer where spherical green pellets are formed. After these reached a size of approximately inch they were screened to remove undersized pellets and then fed to a rotary kiln together with 25 to 100 parts of solid reductant and a small amount, e.g., 2 to 6 parts of limestone. Lignite, lignite char, coke, anthracite coal are among solid reductants which have been employed successfully in the laboratory or on a pilot plant scale.

The materials were heated in the rotary kiln by natural gas, whereby the green ore pellets were reduced and hardened simultaneously. The extent of the reduction and the degree of hardness are a function of the residence time in the kiln and the maximum operating temperature. We have discovered the maximum temperature should not exceed 1250 C. and preferably not more than 1150 C. Ordinarily /2 hour or more at the maximum temperature is required.

The reduced and hardened pellets were protected from contact with air as they discharged from the rotary kiln and were cooled to ambient temperatures in a dry, nonoxidizing atmosphere. The cooled mixture of reduced pellets, ash, lime and excess reductant was screened. The minus Ar-in. plus At-in. fraction was sent to a dry, lowintensity magnetic separator and the magnetic fraction recovered as finished product. The non-magnetic fraction was discarded.

Experimental data Table 1 lists the results of 14 consecutive days of operation utilizing a pilot-sized rotary kiln 3 ft. in diameter by 36 ft. long. The reductants were either lignite or mixtures of lignite and anthracite coal. The rates at which these materials were introduced to the kiln on an hourly basis are also given. Green iron ore pellets (about /2-in. diam.) containing about 9 percent moisture and with an iron content of 63.5 percent (dry basis) were also introduced to the kiln at the rates indicated. A high degree of metallization was achieved and the total iron content of the product pellets was in many cases over 80 percent. The temperature range in the kiln was from 400 C. at the feed end to l at the discharge end. The pellets were in the kiln 1 /2 hours to 2 hours with approximately /2 hour in the hottest zone.

TABLE 1.-RESULTS OF FOURTEEN DAYS PREREDUCED PELLET PRODUCTION Days of test (24 Lignite Anthracite Wet pellets Compressive Percent Total Percent Fe", Total Fe hour periods) (lbs/hr.) (lbs/hr.) (lbs/hr.) strength (1105.) Fe Metallic Fe EXAMPLE 2 tures employed were substantially higher than would be 20 required industrially. However, high blast temperatures A number of reduced and unreduced ore pellets were are desirable in any installation as a means of realizing smelted alone or in various combinations in the Bureau of the ultimate in productivity and coke savings with the Mines experimental blast furnace at Bruceton, Pennsylreduced burden materials. The blast air volume was convania. This furnace had a4 f0ot diameter hearth, 20-foot stant at a nominal 800 s.c.f. m.; it contained 7.0 grains stack and 39-inch bosh. Because the furnace is small, cool- 25 moisture per cubic foot and a constant 2.1 volume pering losses relative to the heat input were large compared to a commercial blast furnace, hence the hot blast temperacent of natural gas. Other data are presented in Tables 2, 3 and 4.

TABLE 2.IDENTIFICATION AND ANALYSES OF MATERIALS USED IN COMPARING PREREDUCED AND OXIDE PELLETS IN THE BUREAU OF MINES EXPERIMENTAL BLAST FURNACE l Analysis, percent dry Description Designation Fe S101 A1 0 CaO MgO Commercial oxide pellets from Hoyt Lakes,

Minn. concentrates A 62.8 8.5 0.45 0.63 0.39 Prereduced pellets from Hoyt Lakes, Minn.

concentrates B 79 .0 11.9 .77 1.08 .58 Commercial oxide pellets from Hilton Mine (Quebec) concentrates C 66.7 2.1 .51 .30 1.71 Partially prereduced pellets from Hilton Mine (Quebec) concentrates D 83.8 3.1 .66 .58 2.06 Highly prereduced pellets from Hilton Mine (Quebec) concentrates E 91.3 3.0 .64 .39 2.40 Calcite .19 .83 .29 54 .6 .64 Dolomite. .67 .93 29 .7 21.6 Limestone. .75 3 .5 .61 52 .3 .56 GraveL... 3.2 81.8 4.9 2.3 .41 Coke ash 6 .8 51.0 29 .6 3.2 1.3

l The proximate analysis of the cokewas as follows: Volatilematter, 0.8; Fixed carbon, 92.0; ash, 7.2 percent.

WITH AN OXIDE PELLET BURDEN, EXPERIMENTAL BLAST FURNACE OPE RATING DATA Base Test No. 1 Base Test No. 2 period 1 period 2 Blast temperature, F 1, 526 1, 561 1, 976 2, 001 Wind rate, s.c.f.m 787 795 791 778 Burden, pounds per A pellets- 598. 4 300. 0 695. 5 B pel1ets 242. 0 621. 2 Average Fe in b 62.82 70. 04 62. 82 78. 98 Burden ratio, pounds Fe per pound coke in charge 1. 67 2.18 1. 77 2. 58 Slag rate, pounds per ton pig iron 643 655 634 738 Top gas:

Excess 002, percent of total 00 8.2 24. 2 Temperature, F 634 497 425 392 C0 percent by volume 17. 1 13. 6 20. 2 7. 0 CO+CO2 1.22 1.73 .96 4.30 Production rate:

Net tons per day (n.t./d.) 16. 2 20. 0 18.1 24. 5 Percent increase 23. 5 35. 4 Coke rate:

Pounds per net ton pig iron 1, 128 900 1, 075 723 Percent decrease 20. 2 32. 7 Natural gas injection:

Cu. ft. per ton pig iron 1, 505 1, 220 1, 344 944 Percent of wind blown 2.1 2.1 2. 1 2. 1 Solution loss, pounds C per ton pig iron 14 26 60 32 oxides and flux decomposition.

TABLE 4.COMPARISON OF VARIOUS REDUCED PELLET BURDENS WITH AN OXIDE PELLET BURDEN, EXPERIMENTAL BLAST FUR- NACE OPERATING DATA Base period Test N o. 3 Test N o. 4 Test No. 5

Blast Temperature, F. 1, 734 1, 700 1, 729 1,750 Wind rate, s.c.f.n1 785 791 778 785 Burden, pounds per charg A pellets 77 100 120 C pellets 519 D pellets 550 256 E pellets 44 590 Average Fe in burden, percent 66.19 80.57 88. 54 86.49 Burden ratio, pounds Fe per pound coke in charge 1. 72 2. 68 3.10 3.07

Slag rate, pounds per ton pig iron 280 243 297 282 Top gas:

Excess 002, percent of total C 11.6 42.4 78. 4

Temperature, F 681 685 633 644 00 percent by volume. 16. 5 10. 2 8. 8 5. 7

OO-Z-COg 1.35 2.61 3.11 5.07 Production rate:

Net tons per day 18.1 25.0 27. 4 28. 0

Percent increase 38. 1 51. 4 54. 7 Coke rate:

Pounds per net ton pig iron 1, 105 732 625 621 Percent decrease 33. 8 43. 4 43. 8 Natural gas injection:

Cu. ft. per ton pig iron 1, 315 948 867 849 Percent of wind blown 2.1 2. 1 2.1 2. 1 Solution loss, pounds 0 per ton pig iron. 73 22 63 72 1 That portion of CO2 in top gas originating from sources other than reduction of iron oxides and flux decomposition.

Table 2 presents analyses of the iron-bearing raw ma- /GRAPH l terials tested in the blast furnace and the designations by g 600 which they are identified in succeeding discussions. All i 500 were commercial products except the experimental preg reduced pellets made as described above and designated 300 B. 2 200 Table 3 compares the results of tests in which the B E: 100 pellets were smelted in competition with commercial E oxide pellets made from the same raw materials. In a g", 20 60 80 100 o base period at nominal hot-blast temperature of 1500 8 Prereduction, percent F. with a burden comprised of A pellets, the production rate was 16.2 net tons per day and 1128 pounds of coke were consumed per net ton. In test 1 which followed immediately thereafter, about half the iron units in the feed were supplied by B pellets so that the average iron content in the burden was 70.04 percent. The production rate rose to 20.0 n.t./d., an increase of 23.5 percent and the coke rate dropped to 900 lb./n.t., a decrease of 20.2 percent. The significance of other details of the operation, listed in Table 2, will be apparent to one skilled in the art and need not be discussed further. A new base period (2) was established with A pellets at a higher hot-blast temperature (2000 F. nominal) and then a burden of all B pellets was smelted. With the reduced pellets, a production rate of 24.5 n.t./d. was realized (35.4 percent increase) and the coke rate dropped to 723 lb./n.t. (32.7 percent decrease). Note also that as the metallic iron content of the burden increased the top gas became richer in CO.

Similar experiments were made with commercial reduced pellets (D and E) which were partially, and highly reduced respectively. After establishing a base period with oxide pellets (A and C), tests 3, 4 and 5 were conducted in which all or part of the burden was made up of reduced pellets. Results are given in Table 4. Again productivity made a spectacular leap as the iron content of the burden went up and, consequently, coke consumption, declined. The highest production rate was 28.0 n.t./d. (54.7 percent better than the base period), with a coke rate of 621 (43.8 percent decrease). All the statistics are presented in Table 4.

Percent prereduction (percent oxygen removed from the ore or pellets prior to use in the blast furnace) was plotted against the savings in coke as compared to that resulting from smelting unreduced pellets. The resulting plot, Graph 1, is a straight line through the origin with the equation C=k R where C is the savings in coke in pounds per ton of pig iron produced, k is 5.7 and R is the percent prereduction.

GRAPH 2 Prereduetion, percent Alternate ways of expressing these equations are Production rate increase, percent Fer Fer

where C, P, k; and k are as given previously, Fe is the iron content of the prereduced ore and Or is the oxygen chemically combined with Fe,.

The above data were obtained with a constant absolute volume of blast air, moisture and natural gas; however, the wind rate per pound of carbon gradually diminished. The blast temperature varied from a nominal 1500 F. to 2000 F. Hence, the relationships shown in Graphs 1 and 2 are independent of hot blast temperature, provided that blast temperature in the base period with unreduced burden is substantially the same as the trial periods with metallized burden. These relationships offer the blast furnace operator a means for determining the amount of coke required for most economical operation and the increase in metal production to be gained from replacing the conventional iron oxide with a wide range of metallized burdens. Although the data were derived in the Bureaus experimental blast furnace, the findings are valid for an industrial furnace operating under comparable conditions. In placing a partially reduced burden on the furnace, the operator may estimate the decrease in initial coke requirements with the equation C=5.7R and the expected production rate increase from P=0.6R. Subsequent operation may establish somewhat diflerent relationships depending on furnace geometry and operating characteristics. Similarly, for furnaces operating under other conditions (oi1-, coal-, or oxygen injection, higher wind rates, etc.) these equations will guide the operator for initial trials with reduced burdens. Thereafter only a few smelting tests would be required to determine the value of the constants k and k and thus determine the linear relationships between C, P and R.

A principal reason for the additional gains realized from the metallized burden is that as the degree of reduction increases, the blast furnace operation approaches that of the cupola. In a blast furnace virtually all the coke carbon is partially combusted according to the equation:

which yields 53,280 B.t.u. per pound mol of carbon. In a cupola furnace most of the carbon is burned completely: C+O CO which yields 169,290 B.t.u. per pound mol of carbon and the heat energy is consumed in melting rather than smelting. Hence, to realize the maximum benefits of the reduced burden, the blast furnace operation must trend toward that of the cupola, e.g., a higher air-to-carbon ratio. In these tests as the degree of prereduction went from to 87, the blast air at a nominal 800 s.c.f.m. went from 454 to 473 s.c.f. per pound of coke. It is prognosticated that as the cupola operation is approached and the proportion of coke in the blast furnace becomes lower and lower, some of the coke is burned to CO and leaves the tuyere zone without reduction to CO. In some of these tests, with the highly reduced burdens, the CO content of the top gas was higher than could be accounted for by the CO in the flux and the formation of CO from indirect reduction in the stack. This can be explained, as noted above, by burning carbon at the tuyeres to CO or carbon 8 deposition in the stack (2CO C+CO In either event, more energy would be available in the tuyere zone.

The data presented were derived through the use of iron oxide pellets and reduced pellets of roughly spherical shape which had been pelletized in conventional discs or drums. These data are illustrative of the manner in .which the invention may be practiced but are not exclusive. Those skilled in the art will recognize that other reduced agglomerates such as those prepared from briquetted or extruded fine iron ore and concentrates may also be satisfactorily utilized. In our experiments, the blast furnace was operated with injections of both natural gas and moisture with the air through the tuyeres. The furnace could also be operated with injections of bituminous, crude oil, other hydrocarbons, or oxygen either in addition to, or in place of, those injectants actually employed in the experiments.

What is claimed is:

1. A method for the production of pig iron from iron ores by reduction employing coke and limestone in a blast furnace, and wherein an iron ore partially prereduced to a higher total iron content prior to charging said ore to a blast furnace is employed, the improvement comprising adding coke in an amount in lbs. per ton of hot metal produced which is less than the amount required for a non-prereduced ore in the blast furnace by k R, where k is a constant having a value of about 5.7 and R is the percentage of prereduction of the iron ore, whereby the percentage increase in productivity in a given time over pig iron produced in the blast furnace employing non-prereduced ores is k R wherein k is a constant having a value of about .6 and R is the percent of prereduction'of the iron ore.

2. In a method and improvement as set forth in claim 1, the further improvement which comprises injecting natural gas and moisture into the blast furnace.

References Cited by the Examiner UNITED STATES PATENTS 1,489,361 4/1924 Moxham 38 1,941,983 1/ 1934 Gudrnundsen 7542 2,544,697 3/1951 Lewis 7541 X 2,605,179 7/1952 Lindemuth 7541 X DAVID L. RECK, Primary Examiner.

H. W. TARRING, Assistant Examiner. 

1. A METHOD FOR THE PRODUCTION OF PIG IRON FROM IRON ORES BY REDUCTION EMPLOYING COKE AND LIMESTONE IN A BLAST FURNACE, AND WHEREIN AN IRON ORE PARTIALLY PREREDUCED TO A HIGHER TOTAL IRON CONTENT PRIOR TO CHARGING SAID ORE TO A BLAST FURNACE IS EMPLOYED, THE IMPROVEMENT COMPRISING ADDING COKE IN AN AMOUNT IN LBS. PER TON OF HOT METAL PRODUCED WHICH IS LESS THAN THE AMOUNT REQUIRED FOR A NON-PREREDUCED ORE IN THE BLAST FURNACE BY K1R, WHERE K1 IS A CONSTANT HAVING A VALUE OF THE IRON ORE, R IS THE PERCENTAGE OF PREREDUCTION OF THE IRON ORE, WHEREBY THE PERCENTAGE INCREASE IN PRODUCTIVITY IN A GIVEN TIME OVER PIG IRON PRODUCED IN THE BLAST FURNACE EMPLOYING NON-PREREDUCED ORES IS K2R WHEREIN K2 IS A CONSTANT HAVING A VALUE OF ABOUT 06 AND R IS THE PERCENT OF PREREDUCTION OF THE IRON ORE. 